1. Field of the Invention
This invention is a cooking method using a novel container formed from a blank of a novel material (i.e., a laminate of at least a layer of polyethylene terephthalate and nonbrowning paperboard). This invention is also a method of making this novel laminate including the step of heating, at critical temperatures, an uncoated surface or surfaces of the paperboard to improve adhesion between it and the polyethylene terephthalate material extruded thereon.
2. Description of the Prior Art
The following prior art is of interest to the laminate and its method of manufacture of this invention:
U.S. Pat. No. 3,402,086 to Smith et al. shows a process for hot-melt extrusion coating of paperboard or paper with a random copolymer of ethylene containing .alpha.,.beta.-ethylenically unsaturated monocarboxylic acid wherein the paper is preheated by hot rolls to a temperature within the range of 80.degree. to 250.degree.C. prior to being melt coated.
U.S. Pat. No. 3,498,865 to Paquin et al. shows a continuous process for coating paper with a film of extrudable plastic, such as polyethylene, wherein the air in the region of convergence of paper and plastic film is ionized. The specification discloses that the paper which contacts the molten polyethylene is heated by use of steam-heated rolls to a temperature of 65.degree. to 115.degree.C. Infrared or other heating elements may also be used for heating the paper.
Canadian Patent No. 529,361 to Fields shows a process for extrusion coating a synthetic linear polymer having a sharp melting point onto a sheet. Polyesters are disclosed as useable materials for the coating and paper is included as a material that can be used for the sheet.
Canadian Patent No. 781,424 to Marshall discloses a method of coating a porous web with a continuous film of a thermoplastic polymer wherein the porous web is preheated before the application of the molten film of the polymeric material. Porous paper may be used as the porous web material and polyesters may be used for coating the porous webs.
British Patent No. 1,119,804 to Rohm & Haas Company discloses a laminate comprising a substrate such as a sheet of paper and a layer of polyhexamethylene terephthalate which has been melt-extruded thereon.
A prior art patent of interest to the container and cooking method of this invention is U.S. Pat. No. 3,276,900 to Funck, which shows the use of a paperboard component in a container used in cooking foods at relatively high temperatures.
Another patent of interest in the food container art is U.s. Pat. No. 3,316,102 to Doll et al. which shows a food tray blank, container and package, and which blank includes means for locking parts of the container together to hold it in erected position without use of adhesives or mechanical fasteners.
U.S. Pat. No. 3,550,835 to Persson is also exemplary of the container and cooking arts. Such patent teaches a package especially for food products, which package is a tray erected from a paperboard blank which has at least one side a fluid-tight plastic coating, i.e., polypropylene, which is adapted to be sealed and withstands heating to about 305.degree.F. While this package is suitable for relatively low cooking temperature use, it cannot withstand heating at the high temperatures above 325.degree.F. and to about 400.degree.F., which temperatures are those most commonly used in the precooked, convenience food areas of cooking.